Load sensitive mats incorporating electrical switches are well known and are generally referred to as "switchmats". For example, switchmats are conventionally located adjacent to entry doors of buildings such as supermarkets, airports and other public places so as to lie in the path of pedestrians approaching doors. A pedestrian stepping upon the mat would close a normally open switch contained in the mat to actuate a mechanism, for example, for opening the door automatically. Switchmats are also used in industrial environments to limit access to dangerous machinery or control the operation of machinery. In particular, a switchmat may be employed to actuate a safety circuit preventing operation of a machine when a person enters a dangerous zone.
Examples of known pressure-sensitive switches and switchmats are disclosed in British Pat. Nos. 392 936, 1 185 862, 1 209 564, 1 351 911, 1 358 006, 1 369 174, 1 454 805, 2 064 222, 2 083 858 and 2 088 637, U.S. Pat. Nos. 1,775,755, 2,951,921, 3,722,086, 3,812,313, 3,828,277, 4,037,069 and 4,105,899, French Pat. Nos. 1 416 570 and 2 431 178 and German Offenlegungsschrift No. 2 148 760.
Switchmats generally comprise first and second electrically conductive members, which are normally planar, separated by non-conductive material and optionally include one or more intermediate conductive elements positioned between the first and second conductive members. The switchmat is normally constructed without a conductive path between the first and second electrically conductive members to provide an open electrical circuit. The switchmat is subjected to load, when the electrically conductive members are caused to come into electrical contact with each other, to provide a closed electrical circuit. Such contact is established by compression of the non-conductive material and/or bowing of one or both of the electrically conductive members, such that a conductive path is established between the electrically conductive members, optionally via intermediate conductive elements. When the load is removed, the electrically conductive members return to their spaced position.
Switchmats are not sufficiently durable to withstand use in an industrial environment for long periods of time. The switchmats used in industry may be subjected to severe overloads, e.g. the dropping of a heavy object or passage of a hand cart or truck. Such physical abuse of a switchmat may result in the complete failure of its switch mechanism. A need exists for a "failsafe" switchmat which will be rendered inoperative, with the switch permanently in the closed position, by providing a permanent conductive path between the conductive elements to complete an electrical circuit, when its switch mechanism is damaged.